Organosiloxane polymers are prepared on an industrial scale using two basic processes. The most widely practiced process is known as the equilibration process which involves the catalytic rearrangement of siloxane bonds to form an equilibrated mixture. The term equilibration is used to describe the phenomenon which exists when the ratio of linear organosiloxane polymers to cyclic organosiloxane oligomers is maintained at a constant value. When dimethylsiloxane is polymerized, equilibrium is reached when the ratio of linear polymer to cyclic oligomers is about 86:14 on a weight-weight basis. A constant ratio of linear to cyclic organosiloxanes is always reached even when the starting material is a cyclic organosiloxane, a mixture of cyclic material and linear material or an all linear monomer or oligomer.
Equilibration is reached by the use of a wide variety of acidic or basic materials as catalysts. During the equilibration process, a constant breaking and forming of siloxane bonds takes place until the equilibrium point is reached. The massive breaking and forming of siloxane bonds permits the use of chainstoppers which will react to form a terminal non-chain extending group on the end of the polysiloxane molecule. The cyclic oligomers are removed from the reaction mixture at the end of the equilibration process by a stripping process after deactivation or removal of the catalyst. The formation of cyclics is a substantial drawback because it adds to the cost of the process and extends the time necessary to complete the processes.
An alternative process for producing organosiloxane polymers is condensation which promotes the head to tail condensation of silanol terminated siloxane monomers or oligomers. The condensation processes depend on the removal of water to form higher molecular weight polymers. Cyclics are not produced in the condensation process. Useful condensation catalysts include phosphorus nitrogen compounds (PNC). Relatively mild acids and bases have also been used as condensation catalysts. Strong acids and bases have been used as condensation catalysts at temperatures that will not result in equilibration and the formation of cyclic organosiloxanes. The general procedure which is used in the condensation process is to combine one disilanol monomer or oligomer with an acidic, basic or PNC catalyst and, after the desired polymer has reached the desired molecular weight, the polymerization is terminated. The reaction may be terminated by deactivating the catalyst using a suitable alkaline or acidic material. The neutralization will prevent further polymerization and will permit the use of the polymer without further purification. In the case of PNC catalysts, deactivation occurs when the product is heated above 160.degree. C.
In a copending application Ser. No. 08/092,450 filed Jul. 15, 1993, Applicants described a novel process which is characterized as a disproportionation reaction. Applicants discovered that if two or more M and D containing polymers, which have different molecular weights, are combined at weight ratios of from about 1:99 to 99:1 or more preferably from 5:95 to 95:5 in the presence of a condensation/disproportionation catalyst, such as a linear phosphonitrilic chloride, an extremely fast and complete siloxane disproportionation reaction takes place between M and D containing polymers. The reaction results in the formation of a lower molecular weight product than one of the two starting materials without the formation of substantial amount of cyclics.
The linear organopolysiloxanes prepared by the above systems can be crosslinked in various ways to produce cured silicone materials. Some of the previously known methods, however, require expensive platinum, vinyl and hydride groups, or toxic materials such as tin compounds which are harmful to the environment. Other known silicone curing systems rely on the use of organoperoxides. Such peroxides decompose during cure to produce undesirable byproducts. These byproducts must be removed from the rubber product, and generally enter the atmosphere in a process known as post baking.